In recent years, as for a 3-frequency branching circuit in mobile communication equipment, attention is focused on a system of selecting a transmitting and receiving frequency of three frequencies in the systems of GSM, DCS and PCS with one portable telephone or the like due to the expanding user base or globalization of the systems, and moreover, attention is paid to possibility of implementing a smaller size and a lower price as to the mobile communication equipment such as the portable telephone.
Hereafter, an example of the 3-frequency branching circuit in the past will be described by referring to the drawings. Moreover, FIG. 56 shows an equivalent circuit diagram of the 3-frequency branching circuit in the past.
In FIG. 56, a transmitting circuit side 2301 as one side has an anode of a first diode P2301 connected thereto via a first capacitor element C2301, and a contact A has a cathode connected thereto.
Furthermore, the anode side of the first diode P2301 has a control terminal 2302 connected thereto, and the control terminal 2302 plays a role of switching a transmitting and receiving signal of one of the 3-frequency branching circuits.
In addition, a receiving circuit side 2303 as one side has an anode of a second diode P2302 connected thereto via a second capacitor element C2302, and a first strip line L2301 is connected to one end of the anode side of the second diode P2302 having a cathode connected to its ground side, and then the other end of the first strip line L2301 is connected to the contact A.
Furthermore, a transmitting and receiving circuit 2304 as one side connected to the contact A is connected to an antenna terminal 2305 via a low-pass filter and a third capacitor element C2303.
In addition, a transmitting circuit side 2306 as the other side has an anode of a third diode P2303 connected thereto via a fourth capacitor element C2304, and a cathode is connected to a contact B. Furthermore, the anode side of the third diode P2303 has a control terminal 2307 connected thereto, and the control terminal 2307 plays a role of switching the transmitting and receiving signal of one of composite switches.
In addition, a contact C on the other receiving circuit side has an anode of a fourth diode P2304 connected thereto via a fifth capacitor element C2305, and a cathode is connected to its ground side, and a second strip line L2302 is connected to one end of the anode side of the fourth diode P2304, and then the other end of the second strip line L2302 is connected to the contact B.
Furthermore, a transmitting and receiving circuit 2309 as the other side connected to the contact B is connected to the antenna terminal 2305 via a high-pass filter HPF and the third capacitor element C2303.
Furthermore, a high-frequency receiving circuit side 2308 has an anode of a fifth diode P2305 connected thereto via a sixth capacitor element C2306, and in addition, a control terminal 2311 is connected to the anode side of the fifth diode, and the control terminal 2311 plays a role of switching two receiving signals of the 3-frequency branching circuit.
In addition, the other high-frequency receiving circuit side 2310 has an anode of a sixth diode P2306 connected thereto via a seventh capacitor element C2307, and a cathode is connected to its ground side, and a third strip line L2303 is connected to one end of the anode side of the sixth diode P2306, and then the other end of the third strip line L2303 is connected to the contact C.
Operation of the 3-frequency branching circuit constituted as above will be described.
First, the cases of transmitting and receiving a low frequency will be described.
In case of transmitting the low frequency, the first diode P2301 and second diode P2302 will be in an on state by applying a positive voltage to the control terminal 2302.
At this time, the capacitors C2301, 2302 and C2303 cut a DC component, and so the current does not run to each terminal. In addition, a transmitting signal transmitted from a transmitting terminal 2301 is not transmitted to the receiving side because impedance of the first strip line L2301 becomes an infinite size due to the second diode P2302 connected to the ground side.
For that reason, the transmitting signal is transmitted to the antenna terminal 2305 via the low-pass filter LPF.
Next, on receiving, the first diode P2301 and second diode P2302 are in an off state in order to apply no voltage to the control terminal 2302 so that the receiving signal is transmitted from the antenna terminal 2305 to the receiving terminal 2303 via the low-pass filter.
Next, the cases of transmitting and receiving a high frequency will be described.
The third diode P2303 and fourth diode P2304 will be in the on state by applying a positive voltage to a control terminal 2307.
At this time, the capacitors C2303, 2304 and C2305 cut a DC component, and so the current does not run to each terminal. In addition, the transmitting signal transmitted from a transmitting terminal 2306 is not transmitted to the receiving side because the impedance of the second strip line L2302 becomes the infinite size due to the fourth diode P2304 connected to the ground side.
For that reason, the transmitting signal is transmitted to the antenna terminal 2305 via the high-pass filter.
Moreover, on receiving, the third diode P2303 and fourth diode P2304 are in the off state in order to apply no voltage to the control terminal 2307 so that the receiving signal is transmitted from the antenna terminal 2305 to the receiving side. Thus, it can be transmitted to a receiving terminal side C via the high-pass filter.
Next, in case of receiving on a receiving terminal 2308 as one side, no voltage is applied to the control terminal 2307 on receiving as mentioned above.
At this time, the fifth diode P2305 and sixth diode P2306 are put in the on state by applying the positive voltage to the control terminal 2307, but the capacitors C2305, 2306 and C2307 cut the DC component, and so the current does not run to each terminal.
And the signal transmitted from the antenna terminal 2305 is not transmitted to the other receiving terminal 2310 because the impedance of the third strip line L2303 becomes the infinite size due to the sixth diode P2306 connected to the ground side.
At this time, it is transmitted to the receiving terminal 2308 via the high-pass filter.
Next, when receiving on the other, the fifth diode P2305 and sixth diode P2306 are in the off state in order to apply no voltage to the control terminal 2307 so that the receiving signal is transmitted from the antenna to the other receiving side.
For this reason, the other receiving signal can be transferred from the antenna terminal 2305 to the receiving terminal 2310 via the high-pass filter.
However, there was a problem that, although the above configuration is suited to the TDMA (Time Division Multiple Access) method such as GSM, DCS and PCS, it cannot be used for a system wherein the TDMA method such as GSM and DCS and the W-CDMA (Wide-band Code Division Multiple Access) method such as UMTS are mixed.